Method for monitoring a heater

Abstract

Method for monitoring the electrical integrity of a heater and a novel heater for use in such a method. The heater includes an elongate heating member; an insulating jacket which encloses the heating member; a first electrically conductive member which surrounds the insulating jacket; a separating and insulating member which surrounds the first conductive member; and a second electrically conductive member which surrounds the first conductive member and is separated and insulated therefrom by the separating member. The method includes the step of testing the electrical relationship between the first and second electrically conductive members.

I claim: 
     
       1. An elongate heater comprising (a) an elongate heating member;   (b) an elongate insulating jacket which encloses and insulates the heating member throughout the length of the heater;   (c) a first elongate electrically conductive member which surrounds the insulating jacket throughout the length of the heater;   (d) an elongate separating and insulating member which surrounds and insulates the first conductive member throughout the length of the heater; and   (e) a second elongate electrically conductive member which surrounds the first conductive member and is separated and insulated therefrom throughout the length of the heater by the separating member; said heater being one which is suitable for use in a method for monitoring the integrity of the heater while it is connected to a power supply and for reducing the power supplied to it if it incurs physical damage, said method comprising monitoring the impedance between the first and second electrically conductive members and providing means which reduces the power supplied to the elongate heating member if physical damage to the heater causes the impedance between the first and second electrically conductive member to be less than a predetermined magnitude.     
     
     
       2. A heater according to claim 1, wherein the heating member comprises a plurality of electrical elements which are connected in parallel with each other between at least two elongate electrodes. 
     
     
       3. A heater according to claim 2, wherein the electrical elements comprise a continuous strip of a PTC conductive polymer. 
     
     
       4. A heater according to claim 1, wherein the heating member is a self-regulating heating member. 
     
     
       5. A heater according to claim 1, wherein at least one of the first and second electrically conductive members comprises wire braid. 
     
     
       6. A heater according to claim 1, wherein at least one of the first and second electrically conductive members comprises conductive ink. 
     
     
       7. A heater according to claim 1, wherein at least one of the first and second electrically conductive members comprises shredded metal. 
     
     
       8. A heater according to claim 1, wherein at least one of the first and second electrically conductive members comprises micro encapsulated conducting substances. 
     
     
       9. A heater according to claim 1, wherein the insulating jacket comprises a polymer. 
     
     
       10. A cable according to claim 1, wherein the insulating jacket comprises a micro-encapsulated insulator. 
     
     
       11. A cable according to claim 1, wherein the insulating jacket comprises a self-repairing gel. 
     
     
       12. A cable according to claim 1, wherein the insulating jacket comprises semiconducting materials. 
     
     
       13. A cable according to claim 1, wherein the insulating jacket comprises mechanically breakable beads. 
     
     
       14. A cable according to claim 1, wherein the separating and insulating member is an electrically weaker insulator than the first insulating jacket. 
     
     
       15. A cable according to claim 9, wherein the separating and insulating member comprises a polymer which is a less effective electrical insulator than the polymer of the insulating jacket.